Valve ends that are expanded into a rounded knob are termed capitate. Note that the shape terms subcapitate and subrostrate are quite close, and that many taxa can be described as possessing subcapitate to subrostrate ends. In general, capitate (and subcapitate) ends are more expanded than the most narrow point, while rostrate (and subrostate) ends are not more expanded than the most narrow point.
Barber, H.G. and Hayworth, E.Y. (1981). Guide to the Morphology of the Diatom Frustule. Freshwater Biological Association, 112 pp.